THE ANGIOSPERMAE 1201 



cases, if the thickened layer appears to be an exothecium it is because the 

 epidermis has disappeared or has been suppressed. The truly porose 

 anthers of Erica (Fig. 1172) and those of some very reduced water plants, 

 such as Zannichellia and Naias, which burst by swelling, have neither 

 endothecium nor exothecium. 



On the general question of the morphology and evolution of the stamen 

 we will limit ourselves to a few concluding words. The classical and 

 straightforward view of the stamen regards it as a modified foliar appendage, 

 though not, of course, a modification of the existing type of green foliage 

 leaf. It is true that stomata are present on the stamens, as on all other floral 

 organs, not even excepting the internal wall of the ovary, in some flowers, 

 but stomata are not exclusively foliar structures. What their presence does 

 suggest, however, is that the organs bearing them had in the past a photo- 

 synthetic function which the stamens, at any rate, have now quite lost. 



There is, on the other hand, a school of thought which traces in the 

 " perfect " flower the result of a process of condensation from an " inflores- 

 cence " of naked sporangia, and in the opinion of such morphologists both 

 stamens and carpels are regarded as descended from systems of branched 

 axes bearing sporangia. Wilson has argued ably in favour of a branched 

 ancestry for the stamen, the nearest analogue to which at the present day 

 would be the dichotomously branched stamen of Ricinus. These inter- 

 pretations are bound up with the larger question whether the sporangia of 

 the Angiosperms are stachyosporous or phyllosporous, that is, whether 

 their sporangia are axial in origin or are borne on sporophylls. Lam has 

 stressed the importance of this distinction in the Pteridophyta, where it 

 divides the whole phylum into two series, and has argued in favour of its 

 persistence in the Spermatophyta. Acceptance of the former view naturallv 

 involves rejection of the foliar, i.e., sporophyll, interpretation of stamens. If 

 it were clearer what constitutes a sporophyll or what, in terms of the Telome 

 Theory, is exactly the diflPerence between an axis and an appendage, such 

 questions might have more weight. It may be true that in the course of 

 evolution from a primitively telomic structure, some fertile telomes have 

 passed through a foliar modification, while in other lines of evolution they 

 have not, but this is by no means a fundamental diflference and we may 

 perhaps feel that the distinction between leaf and shoot, as between sporo- 

 phyll and sporangiophore, is liable to be overstressed and to savour too much 

 of the old formalistic morphology. 



Of fossil evidence on the evolution of the stamens there is really none. 

 A gulf exists between the microsporophylls of the Gymnosperms and those 

 of the Angiosperms, and comparisons in that direction do not help us much. 

 The story goes a long way back and it is perhaps among the synangia of some 

 of the Medullosaceae that the clue to the starting point mav one dav be 

 found. 



